Set registry, disable sandbox + autoOptimise

Sandbox and auto-optimizing make builds unnecessarily slower. 'hey gc'
will optimise the store anyway.

bin/hey

@@ -63,6 +63,7 @@ function main {
     if [[ $EUID == 0 ]]; then
       echo "Cleaning up your system..."
       _do sudo nix-collect-garbage ${@:--d}
+      _do nix-store --optimise
       # nix-collect-garbage is a Nix tool, not a NixOS tool. It won't delete old
       # boot entries until you do a nixos-rebuild (which means we'll always have
       # 2 boot entries at any time). Instead, we properly delete them by
@@ -73,7 +74,6 @@ function main {
       echo "Cleaning up your user profile..."
       _do nix-collect-garbage ${@:--d}
     fi
-    _do nix-store --optimise
   }
 
   ###

flake.nix

@@ -88,8 +88,10 @@
                   binaryCachePublicKeys = [
                     "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
                   ];
-                  autoOptimiseStore = true;
-                  useSandbox = true;
+                  registry = {
+                    nixos.flake = nixos;
+                    nixpkgs.flake = nixos-unstable;
+                  };
                 };
                 system.configurationRevision = mkIf (self ? rev) self.rev;
                 system.stateVersion = "20.09";